SDS and guanidine form an extremely insoluble soap. In fact, one
procedure for removing SDS from solution involves sequential
precipitation with KCl and then guanidine (if you go with guanidine
immediately, you coprecipitate variable, but often major, amounts of
your protein). If you contemplate SDS-PAGE, you're far better off using
urea or SDS-urea as denaturants. Just don't boil the solutions at high
pH-you may carbamylate amino groups. Urea is directly compatible with
SDS-PAGE; the only artifacts result from big osmotic differences
between lanes if, for example, your standards do not contain urea.
Malcolm Moos Jr., M.D., Ph.D.